Study On Preparation And Properties Of Ni-Al/Ni-Cr Composite Coatings On NdFeB Surface By Pulse Plating

NdFeB magnetic material is widely used as functional material because of its excellentmagnetic properties in modern industry and electronic technology. But neodymium is one ofthe metal elements with high chemical activity, and NdFeB material is multiphase structurewith difference electrochemical degree, these characteristics impair the properties of materialwhich easily cause corroded and magnetic properties falling when this material is used in thewarm air, high temperature environment or in the electrochemical environment for a long time.At present, surface protection method is mainly adopted in preventing NdFeB magneticmaterial from corrosion, and electroplating technology is widely application. In particular,pulse electroplating technology on NdFeB magnetic material corrosion protection showsobvious superiority. By using composite electrodeposition, excellent NdFeB surface protectioncoating can be obtained.In this paper, Ni electrodeposits was electrodeposited on NdFeB material surface bysingle pulse electrodeposition method, with low temperature nickel plating bath. The betterpulse electrodeposition parameter was chosen by inquiring the effects of average currentdensity and duty ratio on the properties of the coating, and prepared pulse composite coatingunder this parameter conditions by adding nano Cr powder and nano Al powder in the platingbath. The coating surface morphology was observed by using SEM with energy spectrumanalysis and coating microstructure was analysed with XRD; materials corrosion resistance in3.5%NaCl solution was tested by using potentiodynamic polarization curves and ACimpedance spectroscopy methods; meanwhile the microhardness of coating was tested byusing microhardness tester.The results showed that, the surface of NdFeB with Ni coating by PC electro-depositionwas uniform and dense when average current density was2.0A/dm2, and duty ratio was0.4. Nigrain showed in small grain size with high degree of homogenization in different growthdirections. Ni coating by PC electro-deposition showed better corrosion resistance than that ofby DC electro-deposition in3.5%NaCl solution. The hardness of Ni coating by PCelectro-deposition was higher. When preparing PC NdFeB-Ni-Cr and NdFeB-Ni-Al compositecoating with addition of nano Cr powder and nano Al powder, Cr and Al particles can reducethe preferred orientation of crystal growth and refine the grains. Adding Cr in the electrodeposition solution can speed up the Ni deposition and be conducive to the depositionprocess. Coating corrosion resistance in3.5%NaCl solution was enhanced and protection onNdFeB substrate was better. The addition of Cr or Al nanoparticles in the electrodepositionsolution played the role of hard particles, which increased the hardness of costing accordingly.